Obstacles to fusion energy for civilian purposes

[bobsmith76]

Does all it require to create fusion energy is to heat something up to 8000 C? Is that in principle all that it requires? Does it require any other fundamental tricks?

 

[jedishrfu]

you're creating a plasma at 8000C and you'll need a magnetic bottle to contain it that's one of the problems as they interact and the plasma breaks out of the bottle.

The other way is by zapping a pellet of fusible material dropped into a sphere by lasers from all directions causing a fusion explosion. The debris clouds up the glass that the lasers shoot thru decreasing efficiency.

 

[etudiant]

Getting fusion for a moment is pretty easy, researchers were achieving it routinely as early as the 1950s.
Getting sustained fusion at a high enough level to allow useful energy to be extracted is the problem. We have not yet been able to build a fusion reactor that merely breaks even energetically, nor have we solved the thorny task of engineering a reactor that will be able to sustain fusion reactions for decades.
Our current designs, the ITER project in Europe and the US laser implosion driven approach mentioned by jedishrfu, are both huge, stadium sized devices that maybe hopefully will generate some excess energy once they work. Neither would be anywhere near ready for prime time as an energy source.

 

[Astronuc]

Does all it require to create fusion energy is to heat something up to 8000 C? Is that in principle all that it requires? Does it require any other fundamental tricks?

One won't get much of a plasma at 8000 K. It should be more like 8 keV * 11605 K/eV ~ 93 million K.

Two approaches are inertial confinement whereby a pellet of frozen deuterium or deteurim+tritim is blasted with lasers such that the ablation of the outer layers compresses the heated pellet to the millions of K and high density required for fusion, and magnetic confinement whereby a heated plasma is magetically confined long enough to have a sufficient amount of fusion energy production to exceed the energy input into the plasma.

The problem with magnetic confinement is that the pressures, which are proportional to temperature and particle density, are limited by magnetic field strength, which is an inherent physical limitation of the superconductors and the structure. The d+t reaction is the easist to obtain, but it is handicapped by the fact the 80% of the fusion energy is taken by the 14.1 MeV neutrons, and only 3.5 MeV goes to the alpha particle. More attractive reactions, e.g., d + He3, which is aneutronic, are handicapped by the requirement for higher temperatures, hence lower particle densities, and in the case of d+He3, He3 is rather rare.

Other challenges are heating the plasma efficiently and minimizing the energy losses due to cyclotron radiation, brehmsstrahlung radiation, recombination, and leakage of neutrals, as well as degradation of strucutural materials under high temperature irradiation.

 

[cmb]

Does all it require to create fusion energy is to heat something up to 8000 C?

I'm curious to know where you got this impression from? As per Astronuc, maybe you misread 'eV' for 'C' somewhere. But '8000' is still a weird figure to have ended up with as a starting point.

 

[debra]

It should be easy to create the necessary fusion temperature because there is no theoretical limit (AFIK) to how much laser energy can be put into a small space.
So it is only a matter of concentrating enough laser beams onto one spot.
Cannot understand our delay in this one. It should be possible even with a small laser in a thin enough beam.

 

[Astronuc]

It should be easy to create the necessary fusion temperature because there is no theoretical limit (AFIK) to how much laser energy can be put into a small space.
So it is only a matter of concentrating enough laser beams onto one spot.
Cannot understand our delay in this one. It should be possible even with a small laser in a thin enough beam.

It's not as easy as one thing, particularly the time of multiple laser beams as well as repetitively hitting the same spot at the same time.

Then there is the matter of repetitively hitting small capsules of DT at a rate that produces far more energy than is recycled into the laser system.

It is not trivial.

 

[debra]

Oh, so it looks like practical solution are difficult. Do you think there will be an ingenious easy answer or is it a matter of continuing overcoming hurdles?

 

[Astronuc]

Oh, so it looks like practical solution are difficult. Do you think there will be an ingenious easy answer or is it a matter of continuing overcoming hurdles?

There is no ingenious easy answer, but it is a matter of overcoming hurdles with a clever or ingenious solution.

 

[Newbeak]

Is magnetized target fusion a viable option for a commercial reactor? I was reading about the work being done by General Fusion in B.C. Canada: http://www.generalfusion.com/index.html

 

[BSFusion]

Here are a few obstacles that laser ICF power plants still need to overcome:

1. Manufacturing the cryogenic fuel capsules requires extremely high-quality surfaces, down to the atomic level. This is prohibitively expensive, especially for a power plant that fires several capsules per second.
2. Not only do these BB size targets have to be tracked as they fly through the reactor, but all of the laser beams have to hit them midair from several meters away, a task that for direct-drive ICF requires 50 micron accuracy.
3. The blast chamber must be evacuated several times per second, between shots, to prevent interference with subsequent laser shots.
4. The optics (and walls) must be protected or they will vaporize and fail structurally due to the extreme thermal impulse stresses that result from the intense x-ray, 14 MeV neutrons, and 3.5 MeV alphas. The biggest problem for ICF’s final optics is that there is no scheme yet proposed for either Direct Drive or Indirect Drive that has complete credibility. Optics protection is still one of the weak areas for laser driven ICF.
5. In order to reach the gains necessary for a commercial power plant, self-heating of the fuel by 3.5 MeV alphas is required. Unfortunately, because ICF capsules are small, the alphas escape from the burn zone before depositing their energy. This problem cannot be solved by simply making the capsules bigger, because the blast-chamber constraints would be exceeded by the larger yield.
6. A major obstacle that ICF capsules encounter is turbulent mixing that can quench the fuel and prevent ignition. Rayleigh-Taylor Instabilities (RTI) arise in situations where low-density fluids push into high-density fluids. ICF capsules are vulnerable to this type of instability twice during their compression, 1st at the start, when low-density, high-pressure plasma pushes the higher-density tamper material inward, and 2nd when the implosion stagnates and high-pressure fuel pushes the higher-density tamper material outward. In 2009, W.J. Nellis of Harvard University wrote a critique of the National Ignition Fusion program. The article was titled “Will NIF work?” It contained the following quotes, “Despite the financial and human resources and time spent on NIF, the key condensed matter and materials physics issues of the fuel capsule remain unsolved and the R-T instability continues to be the limiting feature of NIF performance.” “While both the physics of R-T growth and the equations of state of DT (hydrogen) and shell material must be known, the R-T instability is by far the major issue…” “It is R-T spikes that grow from such R-M instabilities under high accelerations at later times that are the show-stopper of ICF.” “Computational simulations for more than 35 years have provided no insight into eliminating the R-T instability.” If large pellets could be imploded with increased laser energy, then the thickness of the mixed region and loss of the hot-spark region would become less serious, but a method to achieve such an implosion must first be found.
7. Attaining tritium self-sufficiency might be fusion’s most difficult challenge. There are no practical, external sources of tritium, so fusion plants must breed their own; current inventories are extracted from heavy-water reactors, which produce 1.7 kg/year, and this supply will peak around 2025 at a mere 27 kg, enough to run a 1 GW fusion plant for six months. The main source of tritium is expected to come from breeding by capture of fusion neutrons in lithium contained in a blanket surrounding the fusion core. Because any lost neutrons would result in a tritium-breeding ratio less than 1.0, a neutron multiplier is usually employed with the hope of overcoming the negative effects of having a front wall ahead of the breeding blanket. Ironically, for an unlimited fuel source, fuel supplies (short-term) will determine how quickly fusion plants can be brought online and how effective fusion can be toward addressing our current worldwide energy crisis. Two key parameters, that influence how long it takes to produce enough tritium for a subsequent plant’s start-up are the fractional burn-up rate (low burn-up fractions require extra fuel-cycles, leading to higher retention-times and greater fuel losses through beta decay) and the trapped (inside the blanket) inventory size.
8. Because laser-compressed ICF capsules obtain temperatures & pressures existing in the cores of stars, they also obtain stellar pressures. Without the weight of an entire star to confine it, ICF plasma disperses rapidly (~0.1 ns) into the vacuum, so that only a small fraction of the fuel gets burnt.
9. A large portion of the laser energy is wasted, backscatter and bremsstrahlung.
10. The ICF rocket compression scheme is not very inefficient (5% - 15%); most of the energy is carried away from the target area by high-energy outgoing ablation material. The peak efficiency of an ablation-driven rocket is typically a factor of 4 or more smaller than that of an ideal rocket because the exhaust is continually heated by the incident flux driving the implosion.
11. High-energy x-ray measurements indicate that up to 50% of the absorbed laser light ends up in hot electrons. The presence of these high-energy electrons, which generally have a temperature of 50-60 keV, make it difficult to achieve the high density compression that is required for a successful burn.
12. Most mainline systems (except for liquid-metal-wall ICF reactors, such as HYLIFE) have steel first walls, which are necessary to maintain a good quality vacuum and to endure the intense x-ray and neutron radiation. The first walls of all such reactors will be highly radioactive (2 to 5 billion curies). In addition, these first walls will require replacement every few years because of neutron-induced damage, either from helium embrittlement or from atomic displacements. Because both neutron energy and neutron population are reduced in the steel first walls of these reactors, neutron multipliers (such as lead or beryllium) or isotopic enrichment of Li-6 are usually required to achieve acceptable tritium breeding ratios. The same applies to magnetic fusion reactor chamber walls. For example, the STARFIRE tokamak walls will have a radioactivity of more than 5 billion curies and must be replaced every four or five years. The significance of this should not be ignored, chamber walls exposed to damage rates of 35 dpa/yr (displacements per atom per year) will require replacement every 5-7 years. Assuming that only the inner structural walls need to be replaced at 30% of the original reactor vessel cost, then about 5% of the plants lifetime must be devoted to replacement activities.
13. ICF laser firing times need to be increased. NIF requires a timeout for cool-down and recovery after each firing. The high precision laser optics that ICF uses must cool for several hours between firings to recover from thermal expansion.
14. Cooling the laser medium is very inefficient. NIF uses external flash tubes that create significant amounts of non-recoverable low level waste heat which must actively be removed (requiring extra energy) from the gain medium.
15. The Halite-Centurian tests in Nevada apparently showed the DT targets might require up to 20 MJ to ignite. Current ICF designs produce less than 3 MJ.